Real Madrid and Brazil forward Kaka has left the club's training camp in the United States and flown to Antwerp, where he will undergo arthroscopic surgery to repair a meniscus injury.
Kaka complained of discomfort in his knee during a routine medical check-up after linking up with the Spanish club following his participation in this summer's World Cup.
Further tests revealed the new injury.
"It is suspected that there may be an injury to the meniscus, which could require arthroscopic surgery," Madrid said on their website this morning.
"Kaka has travelled to Belgium accompanied by club doctor Carlos Diez and will be examined by a specialist in Antwerp."
The Brazil international struggled all last season with injury after moving to Madrid from AC Milan, and could be set to miss around two months this term following his latest setback.
Nevertheless, coach Jose Mourinho insists the club will be able to cope without the 28-year-old.
"If we have to lose him for a while, it won't be a drama - we have other players," he said.
"But we want him to be at 100% so he is happy. He is a player who is much loved by his team-mates - he is one of the best players in the world."
Kaka missed Madrid's 3-2 win over Mexican side America on Wednesday night.